Chapter 111 Tumors at the Foramen Magnum
Regional Challenges
Tumors in the region of the foramen magnum have long challenged surgeons who confront not only the difficulties of diagnosis but also the technical obstacles of a hazardous tumor resection. Historically, determined efforts often ended in respiratory failure and death. In the 1954 series of Love et al., for example, 34 of 74 patients died postoperatively, most often as a result of respiratory failure.1 However, with the advent of MRI and microsurgical technique, and knowledge of the regional anatomy, surgical outcome has greatly improved as reflected by the results of larger, more recent series.2–6
A wide array of tumors, both malignant and benign, arise in the region of the foramen magnum. Collectively they comprise about 5% of all spinal tumors and 1% of intracranial tumors.7 It is useful to divide all foramen magnum tumors into intra-axial, intradural extramedullary, and extradural masses. Each location is associated with a specific group of tumors and special topographic relationships that present unique surgical considerations.
More than 90% of foramen magnum tumors are intradural extramedullary tumors, and most commonly occur ventrolaterally in relation to the spinal cord. The majority of these tumors are meningiomas and neurofibromas, the former being considerably more common than the latter.2,7 Intra-axial tumors (e.g., brainstem gliomas) and extradural tumors (e.g., chordomas) comprise fewer than 10% of foramen magnum tumors.3
The results of surgical management of these tumors have greatly improved over the years, because, with the advent of advanced microsurgical techniques, intraoperative monitoring, and a detailed knowledge of foramen magnum anatomy, these lesions are amenable to safe surgical resection. Given the variable pathologic anatomy in the region of the foramen magnum and the availability of a variety of surgical options with varying advantages and disadvantages, the surgeon must carefully choose an appropriate surgical approach. The rational basis for this selection is outlined in this chapter, with emphasis placed on the problems presented by intradural extramedullary lesions. There has been increasing interest in the radiosurgical treatment of foramen magnum meningiomas.8 Radiosurgery has been employed as a primary tool in patients with comorbidity or advanced age. It has also been used as adjuvant treatment in patients who have incomplete resection or aggressive tumors. In patients with inoperable tumors, conventional chemotherapy and inhibitors of epidermal growth factor receptor (EGFR), farnesyl transferase, cyclooxygenase 2 (COX-2), the protein kinase MEK-1, and the intracellar signaling pathway PI3k/Akt are being investigated.9
History
A foramen magnum tumor was first described by Hallopeau in 187410 in a case report of a 50-year-old woman who presented with spastic upper extremity weakness that progressed to quadriparesis with brainstem signs. The patient eventually died of respiratory failure. Autopsy revealed a foramen magnum tumor, “the size of a small chestnut,” that caused compression of the lateral funiculi of the spinal cord bilaterally.
Although early attempts at surgical removal of foramen magnum tumors were met with disastrous consequences,11,12 Elsberg and Strauss successfully removed a foramen magnum meningioma from a woman, aged 36 years, who presented with Brown-Séquard syndrome.13 Despite several intraoperative episodes of respiratory failure, the patient enjoyed full neurologic recovery postoperatively. The report of this case in 1929 was accompanied by the first systematic evaluation of foramen magnum tumors.
In their classic 1938 treatise, Cushing and Eisenhardt divided foramen magnum tumors into craniospinal and spinocranial tumors on the basis of their predominant anatomic location and associated clinical symptomatology.14 Since that account, several series of patients with foramen magnum tumors have been reported in the literature, with progressive improvement in outcome.1,3–6,15–19
Historically, most lesions have been approached dorsally. However, for ventrally and ventrolaterally located lesions, two other surgical approaches have been employed to minimize retraction of the neural structures. The transoral approach was originally described by Kanavel in 1919 in a report on the transoral removal of a bullet that was lodged between the atlas and the base of the skull.20 Although the technique has been described most commonly as an approach to extradural lesions,21–24 intradural lesions have also been treated this way.25–28 Like the transoral approach, the far lateral approach was originally described to manage other lesions, and in this instance, vertebral and vertebrobasilar artery lesions were managed rather than foramen magnum tumors.29 This approach has since been modified and adopted to deal with tumors in the region of the foramen magnum.30,31
Pathology and Epidemiology
Major features of the largest series of foramen magnum tumors are presented in Table 111-1. Conceptually, tumors in the region of the foramen magnum are best categorized as intra-axial, intradural extramedullary, and extradural tumors, similar to the classification used for spine tumors. Intra-axial tumors are predominantly brainstem gliomas but also include gangliogliomas, anaplastic astrocytomas, ependymomas, and cavernous hemangiomas. Caudal extension of medulloblastomas and hemangioblastomas into the foramen magnum occurs in children and adults, respectively.32 Intradural extramedullary tumors consist mainly of meningiomas and nerve sheath tumors and a much smaller number of epidermoid tumors and paragangliogliomas.32,33
Extradural neoplasms are primarily osteocartilaginous tumors, of which chordoma is, by far, the most common. Chondromas and chondrosarcomas may also arise in this region.32 Occasionally, meningiomas extend extradurally. This type of meningioma is associated with more aggressive pathologic features and clinical course.
This distribution of tumors is reflected in the series by Bruneau and George.3 They reviewed 230 cases of extramedullary intradural and extradural tumors of the foramen magnum (intra-axial tumors were excluded). The intradural tumors that comprised almost 80% of the cases reviewed included meningiomas (60%) and neurofibromas (30%). Fifty percent of extradural tumors were chordomas. The most frequently occurring tumors (in order of decreasing frequency) were meningioma (106 cases), neurofibroma (49 cases), and chordoma (28 cases).
The topography of foramen magnum meningioma is of special interest to surgeons. Three characteristics define the lesion.2,3 First is the compartment of origin. Tumors are divided into intradural (representing the majority of lesions), intra-extradural, and extradural (rare). The intradural lesions are further divided into three groups based on their point of attachment. Ventral tumors are attached to the ventral portion of the foramen magnum (dura, spinal root, or spinal cord) on both sides of the midline; lateral tumors originate between the midline and the dentate ligament; and dorsal tumors have a point of origin dorsal to the dentate ligament.3 Using these strict criteria, George et al. found that among the 106 meningiomas in their series, 56% occurred laterally, 31% ventrally, and 13% dorsally.3,33 Other authors have reported a similar distribution.4–6,16,18 Third, the relationship to the vertebral artery is defined. Tumors arising caudal to the vertebral artery displace the lower cranial nerves upward, tumors arising above the vertebral artery displace the nerves caudally, and tumors spanning the craniocaudal extent of the vertebral artery displace the nerves unpredictably.2,3
The relative rarity of foramen magnum tumors belies their clinical importance. Compared with other CNS neoplasms, foramen magnum tumors occur infrequently: they account for only 5% of all spinal neoplasms and 1% of all intracranial neoplasms. Considering meningiomas alone, those occurring in the region of the foramen magnum account for only 1.2% to 3.2% of meningiomas.6,16,18
Among the large series, the age range of patients with foramen magnum tumors was 2 to 81 years, but the majority of these tumors occur around the fifth decade.1,4–6,16–19,34 The average time between onset of symptoms and diagnosis was 2.5 years. The mean age was 47 years, with a female-to-male ratio of 1.5:1.3 These figures are consistent among authors.4–618 Female predominance of meningiomas in general is also a consistent finding.
Besides neoplasms, other entities can present as foramen magnum lesions and should be considered in the differential diagnosis. Calcium pyrophosphate deposition in the transverse ligament can form a tumor-like mass that compresses the cervicomedullary junction.35 This condition is common in the elderly and rarely becomes symptomatic. It can be diagnosed by CT, which demonstrates calcification around the odontoid. Tuberculosis can also affect the cervicomedullary junction in isolation.36 Although uncommon, this condition should be considered in individuals with systemic tuberculosis, patients from geographic areas where it is endemic, and patients with HIV. A recent series of 29 cases of craniocervical tuberculosis describes frequent destruction of the condyles, clivus and dens, and ventral arch of the atlas. The majority of patients harbored space-occupying soft tissue masses in the epidural and paravertebral spaces, and they were large enough to cause myelopathy in 12 out of 29 cases. Cervicomedullary compression can also occur in craniometaphyseal dysplasia,37 a sclerosing bone disorder characterized by bony encroachment of neural foramina.
Clinical Presentation
Many authors have noted that no signs or symptoms are pathognomonic for foramen magnum tumors.16,38 As early as 1937, Symonds and Meadows observed that “the clinical picture which results from compression of the spinal cord at, or near, the level of the foramen magnum is not always easy of recognition.”19 Indeed, the clinical presentation of foramen magnum tumors usually varies and includes such ubiquitous symptoms as neck pain and limb dysesthesias, which are also associated with several more common diseases. The rarity of foramen magnum tumors may therefore cause them to be overlooked by clinicians.39 Even a series as recent as that of Bruneau and George, which reviews 230 cases of foramen magnum tumors from 1985 to 1995, reports a misdiagnosis rate as high as 13.5%.3
The most common presenting symptoms of foramen magnum tumors, in order of decreasing frequency, are suboccipital or neck pain, dysesthesias of the extremities more frequent in the upper than in the lower extremities, gait disturbance, and weakness more frequent in the upper extremities than in the lower. Other common early symptoms include clumsiness of the hands, bladder disturbance, dysphagia, nausea and vomiting, headache, “drop attacks,” and dizziness.4,5,16,18 Usually a patient presents with a constellation of symptoms. Rarely, the presentation is characterized by brainstem symptoms primarily (e.g., nausea) or pure nerve dysfunction (e.g., hemifacial spasm,40 dysphagia,41 occipital neuralgia42).
Suboccipital or upper cervical pain, probably caused by irritation of the dura and C2 nerve root, is the most common presenting complaint and may precede other symptoms by months or years. C2 distribution sensory loss frequently accompanies the pain, and together, these symptoms should suggest the diagnosis of foramen magnum tumor. In the series by Stein et al.,18 Meyer et al.,5 and Guidetti and Spallone,4 suboccipital or upper cervical neck pain was the initial complaint in 65% to 80% of patients. By the time of admission, 100% of Guidetti and Spallone’s patients complained of neck pain.
Limb dysesthesias are frequently present and may occur in the form of a burning4,6 or cold13,43 sensation; proprioceptive loss is also common.44 Weakness typically accompanies the sensory changes and tends to involve the upper extremities more than the lower, and the ipsilateral more than the contralateral side.16 An unusual feature of the weakness is its occasional association with wasting of the intrinsic hand muscles.45,46 Taylor and Byrnes have cogently argued, on the basis of their own experimental evidence, that foramen magnum lesions produce hand wasting by causing venous obstruction in the upper cervical cord, which leads to venous infarction in the lower cervical gray matter.47 If the motor symptoms dominate the presentation, the combination of hyperreflexia and hand wasting can resemble the presentation of amyotrophic lateral sclerosis.
The initial neurologic examination of the patient with a foramen magnum tumor most commonly reveals weakness, sensory loss, hyperreflexia, Babinski sign, and spastic gait. Typically, the weakness first affects the ipsilateral arm and then evolves over time into a progressive spastic quadriparesis. Sensory loss may involve the modalities of pain and temperature, proprioception, or both. The burning and cold dysesthesias have been mentioned. Other less common, but still frequent, signs include nystagmus (classically, downbeat), accessory nerve palsy, and atrophy of the intrinsic muscles of the hand. Infrequent signs include atrophy of the arms and legs, papilledema, Horner syndrome, and cranial neuropathies involving cranial nerves V, VII to X, and XII.44
The nonspecific signs and symptoms produced by foramen magnum tumors must be distinguished from those occurring in several more common conditions. Although modern neuroimaging has lessened this problem, the neurosurgical literature is replete with examples of tumors in the region of the foramen magnum that were misdiagnosed on initial presentation. Even a recent series of foramen magnum tumors found a 13.5% incidence of misdiagnosis.3 The failure to establish the correct diagnosis most commonly occurs because a foramen magnum tumor has not been included in the differential diagnosis.48 The clinical entities most commonly confused with foramen magnum tumors include cervical spondylosis, multiple sclerosis, syringomyelia, intramedullary tumors, carpal tunnel syndrome, normal pressure hydrocephalus, Chiari malformation, and amyotrophic lateral sclerosis.
Surgical Anatomy
A thorough knowledge of foramen magnum anatomy is critical to safe surgical exposure in this region. For a more detailed review of the microsurgical anatomy, the reader is referred to the elegant anatomic studies of Oliveira et al.,49 Rhoton et al.,50 and Wen et al.51
Osseous Structures
The foramen magnum is formed by the occipital bone, which consists of three parts: basilar, lateral, and squamosal. The basilar part is formed by a fusion between the occipital bone and the clivus. The lateral parts consist of the occipital condyles, which articulate with the atlas. Behind and above the foramen is the occipital squama, whose internal surface is marked by a prominent midline ridge—the internal occipital crest, which serves as the attachment for the falx cerebelli. The ventral margin of the foramen magnum is termed the basion and the opposite margin, the opisthion. The shape of the foramen magnum varies. It is generally oval in shape, and the wider portion is located dorsally.49 It measures on average 35 mm in length and 29 mm in width. The foramen magnum transmits the medulla oblongata; the meninges; the ascending portion of the spinal accessory nerve; and the vertebral, anterior, and posterior spinal arteries.
The occipital condyles are located lateral to the ventral half of the foramen magnum. The occipital condyles are oval, and their inferior surface is convex. They are oriented in a dorsolateral-to-ventromedial direction. They articulate with the superior facet of the atlas, which overlies its lateral mass. The anatomy of the occipital condyles, as it pertains to the transcondylar approach, has been reviewed.52
The hypoglossal canal is located within the occipital bone, ventral to the junction between the ventral and middle third of the occipital condyles.51 The hypoglossal nerve is the only structure that travels through the hypoglossal canal. The jugular foramen is located lateral to the ventral half of the occipital condyles, at the junction of the petrous part of the temporal bone and the occipital bone. It is irregular in shape and has a smaller anterior division and a larger posterior division. The anterior division transmits the inferior petrosal sinus and the glossopharyngeal nerve. The posterior division transmits the vagus and spinal accessory nerves, the internal jugular vein, and the meningeal branches of the ascending pharyngeal and occipital arteries.49
Neural Structures
Cranial Nerves
Any of the lower four cranial nerves may be affected by lesions arising in the foramen magnum. The hypoglossal nerve is formed by a series of rootlets that arise in the ventrolateral sulcus between the pyramid and the olive, along a line that is continuous with the ventral spinal roots. The hypoglossal rootlets course ventrolaterally through the subarachnoid space on their way to the hypoglossal canal, passing dorsally in relation to the vertebral artery. If the course of the vertebral artery is short and straight, there may be no contact between the artery and the hypoglossal nerve. A tortuous vertebral artery, however, may displace the nerve dorsally and medially against the medulla, stretching and damaging its fibers.53 Infrequently, the artery passes through the rootlets of the nerve.49
The glossopharyngeal, vagus, and cranial portions of the accessory nerves all arise in series along the dorsolateral sulcus, between the olive and the tuber cinereum. They exit the skull together through the jugular foramen. The cranial part of the accessory nerve is joined by a spinal part that arises as a series of rootlets between the ventral and dorsal rootlets and ascends through the foramen magnum between the dentate ligament and the dorsal roots. The hypoglossal nerve and, less commonly, the glossopharyngeal, vagus, and accessory nerves may be displaced dorsomedially by a thickened and atheromatous vertebral artery.53
Spinal Nerve Roots
The C1 nerve root often lacks a dorsal rootlet. The accessory nerve frequently contributes a root to the C1 nerve root when the C1 dorsal root, as is commonly the case, is absent. Before exiting the dura mater, the C1 ventral root and the dorsal root, if present, attach to the dorsal caudal surface of the intradural segment of the vertebral artery. The ventral and dorsal roots then exit the dura around the vertebral artery and unite within or just beyond the dural exit.
Vascular Structures
In most individuals, the left vertebral artery is dominant. After ascending through the C1 foramina, the vertebral arteries continue medially with the C1 nerve root along a groove on the rostral surface of the dorsal arch of the atlas, behind the lateral mass. Frequently, this groove forms a complete bony canal that surrounds the vertebral artery.49 Between C6 and C2, the vertebral arteries are thus protected dorsally by the lateral masses. However, as the arteries course dorsal to the C1 lateral mass and enter the region of the foramen magnum, they lose their dorsal bony protection.
Before entering the dura mater, the vertebral artery gives rise to the posterior meningeal and posterior spinal arteries, branches to the deep cervical musculature, and, infrequently, the posterior inferior cerebellar artery. Lang found only a 4% incidence of an extradural origin of the posterior inferior cerebellar artery.54,55
After giving off these branches, the vertebral arteries enter the dura mater just caudal to the lateral edge of the foramen magnum behind the occipital condyles, accompanied by the first cervical nerve and the posterior spinal artery.49
The initial intradural segment of the vertebral artery passes rostral to the dorsal and ventral roots of the first cervical nerve and ventral to the posterior spinal artery, the dentate ligament, and the spinal portion of the accessory nerve. In its ascent along the lower lateral and upper ventral aspect of the medulla, the vertebral artery remains ventral to the lower cranial nerves.7 Variations in this relationship do exist, however, and the vertebral artery may lie dorsal to some cranial nerve rootlets.7 Connections between the hypoglossal nerve, glossopharyngeal nerve, spinal accessory nerve, and C2 cervical root provide the anatomic substrate for various “neck and tongue” syndromes.56
As the vertebral arteries ascend ventromedially along the lateral and then the ventral surface of the medulla, they run adjacent to the occipital condyles, the hypoglossal canals, and the jugular tubercles and then come to rest on the clivus. At or near the pontomedullary junction, the arteries join together to form the basilar artery. The precise point at which the arteries join varies with the size and tortuosity of the vessels.49
The posterior inferior cerebellar artery usually originates from the intradural portion of the vertebral artery just above the foramen magnum, although it rarely arises extradurally at or below the foramen.49 As mentioned, 4% of the posterior inferior cerebellar arteries examined by Lang arose extradurally.54,55 In its course along the ventrolateral and then the dorsolateral medulla, this artery may pass rostrally, caudally, or between the hypoglossal rootlets or above, below, or between the rootlets of the glossopharyngeal, vagus, and accessory nerves.56 Like the vertebral artery, the relationship between the posterior inferior cerebellar artery and the lower cranial nerves is an intimate one and frequently leads to deformation and stretching of the nerves.
After passing through or around the rootlets of the nerves, the posterior inferior cerebellar artery comes to lie dorsal to the glossopharyngeal, vagus, and accessory nerves and then takes a variable course to reach the dorsal medulla, where it bifurcates into a medial and a lateral trunk.56 The medial trunk supplies the vermis and the adjacent cerebellar hemisphere; the lateral trunk supplies the tonsil and hemispheres.
The anterior spinal artery is usually formed by the union of the paired anterior ventral spinal arteries. These arise from the vertebral artery, supply the paramedial ventral medulla, converge, and run caudally along the ventral median fissure of the spinal cord. In a common variant, the anterior spinal artery may arise from a single vertebral artery, supplemented by supply from vertebral radicular branches at C2 or C3.54
